September: Autumn Assortment

Abstract

Edward Barnard first observed this object in 1884 with a 5-in. refractor. In 1924, Edwin Hubble used the 100-in. Mt. Wilson telescope to identify 11 Cepheid variables in this object. In the December 1925 issue of the Astrophysical Journal, Hubble reported that the period–luminosity relationship of these variable stars placed NGC 6822 at least 700,000 light-years distant. This proved that Barnard’s discovery was in fact outside of the Milky Way, and far more distant than the Magellanic Clouds, which were the first confirmed extragalactic structures. Barnard’s Galaxy is an irregular dwarf galaxy with a bar along its north–south axis. At the north border (upper right), several H-II regions can be seen glowing red. In the lower left, a band of stars are grouped into a short arm. Current data places Barnard’s Galaxy about 1.8 million light-years away. Imaging. Barnard’s Galaxy is much more difficult to image than its magnitude of 8.8 and size of 15 arcmin may suggest. First, this target is low in the sky for northern observers, which both introduces more atmospheric effects and limits time for imaging. Second, its light is spread out yielding low contrast. Even with 4 h of exposures from a dark sky site, this image is somewhat disappointing, which I attribute to my latitude of 43° north. If your site is farther south, you should have a better opportunity. Imaging this object requires a dark sky site, so do not try this from the suburbs. Processing. Barnard’s Galaxy is low in contrast and therefore may not benefit from deconvolution or other sharpening methods. After balancing color, boost color saturation to emphasize the H-II regions. If you are at a northern latitude, then you may have gradients from imaging at low altitudes; apply gradient reduction methods if needed (Fig. 9.1).